The Ansa Subclavian (AS)
2.7 EMBRYOLOGY
Asregards the embryology of the peripheral visceral afferent neurons, it is now widely, though not uniformly accepted that their cells of origin develop from the primordial
spinal and cranial ganglia, which are derivatives of the neural crest (pick, 1970).
This theory, first postulated by Kuntz, believes in an outgrowth of the sympathetic cells from the neural tube along the ventral roots and rami communicantes. Within the pale mesoderm, the primordia of the ganglia of the sympathetic trunks are present in the form of aggregates of large and deep stained cells in the lower thoracic and upper abdominal regions along the dorso-Iateral aspects of the aorta. The sympathetic trunks form continuous columns extending from the rostral to the sacral regions of the embryo.
Rami communicantes are also present during this stage. Cells, with an appearance identical to that of the primordia sympathetic trunks, derived partly from the neural tube and partly from the spinal ganglia pass along the ventral and dorsal spinal roots to form the primordial sympathetic trunks (Mitchell, 1953).
According to Kuntz (1953), the cervical sympathetic trunk does not develop from the primordial cervical spinal segments but is formed from a cranial extension of the thoracic sympathetic trunk, as well as by local cell proliferation. The segmentation of the thoracic and abdominal sympathetic, which begins in human embryos at 10mm in length, is not apparent in the cervical region of those of 15mm in length but may clearly be recognized in human embryos of 10mm in length in which individual sympathetic ganglia are connected by distinct internodal rami (Kuntz, 1953).
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fonnation of internodal rami occurs later; the details of this process is not certain, especially in the lower thoracic and lumbar regions, where irregularity in the number and position of SYmpathetic ganglia is a well known fact (pick, 1970). According to Pick and Sheehan (1946), each primordial ganglionic mass divides into a cranial and caudal portion, and the ultimate fate of these portions determines the number and position ofthe SYmPathetic ganglia at various spinal levels ffigure 19]:
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c.
b.
d.
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Figure19:Schema illustrating the development ofpossible variations ofthe ganglionated chainfrom the primordial cell column [Pick, 1970]
a. The two portions merge together again (or really never divide), forming a single ganglion with connections to only one spinal nerve, a type of ganglion truly segmental and most commonly occurring in the upper thoracic regions;
b. A 2nd possibility would be the fusion of the caudal half of one primordial mass with the cranial half of the next lower mass, resulting in ganglia with connections to 2 spinal nerves. Such ganglia are not segmental and are typically in the lower thoracic regions in man.
c. Thirdly, the fusion of positions of more than two primordial masses may take place and result in the formation of ganglia with connections to three or more spinal nerves. This arrangement is exemplified best by SCG and lCG, and is likewise common in the lumbar region in man.
d. A fourth arrangement would result from the persistence of one or both portions of the primordial mass as a separate ganglion, thereby producing additional ganglia or complete duplication. Finally, an entire primordia mass or part of it may seemingly be absent when the rami communicantes usually connected with it are issued directly from the internodal ramus of the sympathetic trunk.
The primordia of the prevertebral plexus appears in the rostral abdominal region, in human embryos of 6mm in length, as a result of the ventral outgrowth of the primordial sympathetic trunks along the lateral aspects of the aorta (Pick, 1970). A similar displacement of cells from the upper thoracic primordial sympathetic trunks towards these preverertebral plexuses takes place at some later stage. Thus in the human embryos of 10mm in length an anlage of prevertebral plexuses, including the splanchnic nerves are quite conspicuous (Pick, 1970). The autonomic ganglia and ganglionic
-
cord become organized into four columns that run the length of the cord: a pair of dorsaValar columns and a pair of ventraVbasal columns (figure 20]. Laterally the alar and basal columns are separated by a groove called the sulcus limitans; dorsally and
Figure20: The formation oftwo ventral motor (basal) columns and two dorsal sensory (alar) columns throughout most ofthe spinal cord [Larsen, 1993J
ventrally they are separated by acute thinning of the neural tissue called, respectively, the roof plate and the floor plate. The cells of the dorsal column develop in association neurons which will interconnect the motor neurons of the ventral columns with the, neuronal processes that soon grow into the cord from the sensory neurons of the dorsal root ganglia. Inmost regions of the cord-at all 12 thoracic levels, at the lumbar levels (Ll-3) and at the sacral regions (S2-4)- the neuroblasts in more dorsal regions of the basal columns segregate to form distinct intermedio-Iateral cell columns [figure 19].
The thoracic and lumbar intermedio-Iateral cell columns contain central autonomic motor neurons of the sympathetic nervous system, whereas the intermedio-Iateral cell column in the sacral region contain central autonomic motor neurons in the parasympathetic system.
Not all peripheral sympathetic neurons are located in the chain ganglia. The peripheral ganglia of some specialized sympathetic pathways develop from neural crest cells that congregate next to major branches of the dorsal aorta. One pair of these prevertebral and pre-aortic ganglia originates from the cervical neural crest and forms at the root of the celiac trunk. Other, more diffuse ganglia develop in association with the superior mesenteric artery, the renal arteries, and the inferior mesenteric artery. These are formed by thoracic and lumbar neural crest cells [figure21].
Cardiac and lung plexus
"Middle cervical gangUon ..Stellateganglion
~~Celiac
ganglion
Superior
mesenteric ganglion AorticorenaJ ganglion
Inferior mesenteric ganglion
- _ . PregangHonic fibers _..
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~"'~.PostgangJionJc fibers
Figure21: Postganglionic fibres emanating from cervical and thoracic chain ganglia follow blood vessels to structures in the head andpharynxandthe heart and lungs
{Adaptedfrom Larsen, 1993]